Frequency modulation system



March 31, 1942.

o. E. DE LAN E FREQUENCY MODULATION SYSTEM Filed July 9, 1957 3Sheets-Sheet 1 INVENTOR 0.5.05 LANGE B) 2 I Af-TORNEY 2a [BEEF/22vvvvvvv v'l -llll FIG. 2

March 31, 1942. p. E. DE LANGE 2,278,063,

FREQUENCY MODULATION SYSTEM Filed July 9, 1937 I 3 Sheets-Sheet 2 L I 67X l0 INVENTOR 0.5. DE LA/VGE W I ATTORNEY March 31, 1942. o. E. DE'LANGEFREQUENCY MODULATION SYSTEM Filed July 9, 1957 3 Sheets-Sheet s FIG. 6'

-I -9 CONTROL TUBE GRID BIAS VDL T small/Tr or menus/var uoouu r50 oscauTOR WITHOUT STAB/U244 T/ON CIRCUIT CHANGE OF PLATE VOLTAGE STABILITY orFREQUENCY MODULA 7:0 OSCILLATOR 0. CHANGE OF PLATE VOLTAGE W/ 7.! STAB/LIZ! T/ON CIRCUIT w m w w. 0 P398 IAN EN 0.5. 0.5 LANCE ATTRNEVPatented Mar. 31, 1942 FREQUENCY MODULATION SYSTEM Owen E.

De Lange, New York, N. Y., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationJuly 9, 1937, Serial No. 152, 74

. 17' Claims.

The present invention relates to frequency modulation of high frequencywaves by signal or other modulating waves.

Among the objects of the invention are, to increase the extent or degreeof modulation, to improve the quality or linearity of the modulation andto increase the frequency stability.

It has been proposed to vary the frequency of an oscillator by changingthe plate impedance of a discharge tube associated with thefrequency-determining circuit such that the plate current is out 'ofphase with the voltage, for example, in phase quadrature with thevoltage in such circuit.

In accordance with this invention, this basic type of circuit has beenmodified to provide frequency modulation by signal waves, and thevarious forms which the improved circuit may take will be indicated inthe following description and illustrated by examples in the drawings.

Fig.1 is a schematic circuit diagram of a circuit embodying theinvention using a single control tube;

Fig. 2 is a similar circuit diagram using a pair of control tubes inpush-pull.

Fig. 3 is a modification embodying a frequency stabilizing circuit inaccordance with the invention; I

Fig. 4 shows idealized curves illustrating the action of this frequencystabilizing circuit;

Fig. 5 shows a modification of the circuit of Fig. 3 and may besubstituted for all that portion of Fig. 3 outside the broken line; and

Figs. 6, 7, 8, and 9 show curves illustrating the performance of thevarious circuits.

Referring first to Fig. 1, the oscillator comprises space discharge tubeI with tuned or tank circuit comprising inductances '2 and 3 andcapacity t, with coupling between inductances 2 and 3. Condenser 5 is astopping condenser. Plate voltage for tubes I and 6 is supplied overconductor l5. Control tube 6 has its grid circuit connected acrossresistance 1 shown serially included in one branch (e. g. the inductancearm) of the tank circuit, one end of which resistance is grounded.Series condenser 9 is included in the grid lead to tube 6 and modulatingvoltages are app-lied to this grid from any suitable signal input If!through transformer H. In the cathode-ground lead of tube 6 are includeda capacity I2 and direct current by-pass in the form of choke coil I3,for a purpose to be described. Screen grid voltage for tube 6 issupplied over circuit M from a positive potential source indicated. Theload coupling circuit is shown at I6.

Except for the signal input coupling, the load coupling and the cathodenetwork l2, 13 this circuit resembles a circuit shown by Travis in Proc.I. R. E., volume 23, No. 10, October, 1935, page 1135, Fig. 8. Inattempting to adapt that circuit to a frequency-modulating system it wasfound that the amplitude variations were larger than could be tolerated.It was found that where large variations of frequency are needed as in afrequency-modulating system it could no longer be assumed that theresistance and inductance of the tank circuit vary by the same amount,as the mutual conductance of the control tube is changed, but that whilethey do change in the same direction the resistance changes the morerapidly and the result is a change in amplitude as the frequency ischanged.

It was found, however, in accordance with this invention that theinclusion of a capacity reactance (l2) in the cathode ground lead of thetube 6 minimized the amplitude changes and permitted satisfactoryfrequency modulation of wide extent. This may be accounted for by thefact that with the condenser l2 in circuit and of the proper size theresistance component of the total impedance represented by the platecircuit of tube 6 in parallel with inductance 3 may be made to vary withvariations of the mutual conductance of tube 6 in such a way as to keepthe ratio of this resistance to the efiective inductance of the totalimpedance constant. This has the effect of keeping the tuned impedanceconstant and therefore the amplitude of the generated oscillations ofvarying frequency constant.

The correct value of capacity i2 does not appear to be critical and itsbest value can be found by trial. one case when applicant was using anR. C. A. type 37 tube as oscillator, an R. C. A. type36 tube as controltube and a mean frequency of about 4 megacycles modulated to an extentof about 1:50 kilocycles, a suitable value for capacity l2 wasmicromicrofarads.

While Fig. 1 has the resistance 1 connected in the grid circuit ofoscillator I and can be used in that way, applicant has found itpreferable to place both resistance 1 and the load coupling circuit inthe plate circuit and in the succeeding figures they will be so shown.Greater freedom from amplitude variations is obtained by connectingresistance 1 in the plate branch of the tuned circuit and this is alsotrue of the load coupling.

The type of operation obtained with the circuit of Fig. 1 is indicatedby the curves of Fig. 6, where curves I and II show amplitude andfrequency pass 13 corresponding variations when capacity l2 and itsbyare absent and curves I11 and IV are curves with l2 and I3 in circuit.It is seen that there is a wide region over which the changes ofamplitude are small and that with proper choice of operating point thiscircuit will produce quite linear modulation of :50 kilocycles withamplitude changes which never exceed 2 per cent or :40 kilocycles withpractically no amplitude change. In this figure the mean frequency wasof the order of 4 megacycles.

An improvement in linearity of modulation can be obtained by use of apair of control tubes in push-pull relation as disclosed in Fig. 2. Inthis figure, tube I is the oscillator tube with tuned circuit comprisinginductance portions 2' and 3' and tuning condenserl' as in Fig. 1. Thetwo control tubes are shown at 28 and 36 with their cathodes connectedto ground through respective cathode networks 22, 23 and 32, 33. Platevoltage is supplied from source l5 forall three tubes. Resistance i1 isin this figure connected between the alternating current ground and theplate of the oscillator so that this resistance is in the plate branchof the tuned circuit rather than in the grid branch. The control gridsof the tubes 26 and 36 are connected through respective condensers 25and 35 to the ungrounded end of resistance [1 whereby these gridsreceive a high frequency voltage from across resistance l1. These gridsare also connected through respective impedances, shown as resistance 24and 34, to the extremities of a speech input coil H the primary of whichis connected to input circuit i0. Impedances 24 and 34 could, ifdesired, comprise radio frequency choke coils. The midpoint of thesecondary winding of transformer II is connected through bias battery-39 to ground. The load circuit coupling is shown at Hi from across theplate coil of the oscillator. The screen grid voltage for tubes 23 and33 is supplied from source 31 with by-pass condenser 38 to ground.

Due to the parallel connection of the control grids of tubes 26 and 36to the resistance l1 the tube 26 appears to the plate path of theoscillator tank circuit as a positive reactance in series with apositive resistance while tube 36 appears to the grid path of the tankcircuit as a negative reactance in series with a negative resistance.The result is that if both tubes have the same mutual conductance theireffects tend to neutralize each other in producing variation of thefrequency. When, however, their mutual conductances are varied inopposite directions by the impressed signal their effects on thefrequency of the generated wave are in the same direction and additive.Each tube is operated at a point where changes in its mutualconductancehave little effect on the amplitude of oscillation so thatthe combination likewise has but a small effect upon the amplitude asthe mutual conductances are varied. It is possible by this circuit toobtain for the same amplitude change a much greater frequency shift thanis obtained with a single control tube.

Typical results obtainable with. the circuit of Fig. 2 (without circuitelements 22, 23 and 32, 33) are illustrated by the curves of Fig. '1 andthese curves also illustrate the improvement secured by includingresistance l1 in the plate branch of the tuned circuit instead of in thegrid branch and taking voltage across plate branch as load voltage aswas discussed in connection with resistance 1 of Fig. 1. It is seen fromthese curves that with the use of the two control tubes and theresistance II in the plate branch of the tuned circuit substantiallylinear modulation is obtainable over a region twice as great as thatillustrated by the curves of Fig. 6 with practically no perceptiblechange in amplitude. When the resistance l1 was included in the gridbranch of the tuned circuit very large changes in amplitude wereobserved. The mean frequency about which the modulation took place inthe case of Fig. '1 was 6.9 megac'ycles. While the circuit elements 22,23 and 32, 33 are shown in Fig. 2 they were not used in the circuit whenthe curves of Fig. 7 were made. As the curves indicate, very goodresults were obtained in this particular circult without the use ofthese circuit elements. The dotted line inclosure around these cathodeimpedances in Figs. 2, 3 and 5 is to indicate that these circuitelements may be omitted although applicant has found them advantageousin some cases with circuits 'of the type shown in these figures.

Fig. 3 discloses the same circuit as in Fig. 2.

with the addition of means for maintaining the frequency of thegenerated oscillations very stable. This latter effect is secured by theuse of a pair of tuned detectors 42 and 43 whose tuned grid circuits 44and 45 are respectively coupled to the inductances of the tank circuitof the oscillator and are tuned one slightly below and the otherslightly above the normal frequency so that these detectors have"crossed characteristics as indicated in Fig. 4. For example, referringto Fig. 4, if the mid-frequency is at point the detector 42 has aresonant characteristic with a slope indicated by line 52 passingthrough the point 50, while the detector 43 has a characteristic with aslope indicated by line 5| passing through the point 50. The effect ofthis will be described presently. Lines 5| and 52 represent idealizedcircuits though it is not at all necessary that these characteristics belinear.

Referring again to Fig. 3, the secondary of the speech input coil II isdivided into portions 46,

v 41 and across the outer terminals are connected a pair of resistances53, 54 and a pair of condensers 55 and 56. The plate of detector 42 isconnected to the upper terminal of winding 43, while the plate ofdetector 43 is connected to the lower terminal of winding 41 as shown inthe figure. The cathodes of the detectors 42 and 43 are connectedtogether and through battery 58 to ground, the polarity being such thatthe cathodes are made negative to ground. The common terminal ofresistances 53 and 54 is connected to the negative pole of battery 51with its positive pole connected to ground. Since the cathodes ofcontrol tubes 26 and 33 are grounded to low frequency and directvoltages battery 51 applies a negative biasing potential to the controlgrids of these tubes (similarly to battery 39 of Fig. 2). Battery 51tends to make the plates of detectors 42 and 43 negative but this isoffset by battery 58 which may be of sufficiently higher voltage ethanbattery 51 to apply resultant normal positive voltage to the detectorplates. Condensers 55 and 56 are small enough to offer high reactance tocurrents of much lower than speech frequency but relatively smallreactance to currents of speech and higher frequencies.

Assuming that the oscillator is operating at its correct normalfrequency (point 50, Fig. 4) and that no modulating waves are beingapplied to the circuit I 0, equal high frequency voltages are applied tothe grids of detectors 2 and 43 producing equal steady plate current inthe detector output circuits and producing the same voltage at points Band D with respect to ground. It is assumed also that battery 51 hasbeen adjusted to produce the desired bias voltage on the control gridsof tubes 26 and 36. If now the frequency of oscillation should rise, theoutput current from detector 43 will decrease and that from detector 42will increase (see Fig. 4) causing the potential of point B and of thecontrol grid of tube 26 to become of point D and control tive andrestoring the oscillator frequency to its normal value 50 as shown bythe curves of Fig. 4.

When speech currents are applied to circuit I they produce variations inthe oscillator frequency as described in connection with Fig. 2. Thedetectors l2 and 43 would endeavor to counteract the changes infrequency but they are prevented from accomplishing this by virtue ofthe condensers 55 and 56 which prevent a voltage of the signalingfrequency from being developed across the points B and D. As noted abovethese condensers are so large that only voltages of frequencies belowthe lowest modulating frequency can exist across the points B and D.Thus the system allows for variation of frequency at rates high enoughfor modulation but at the same time prevents frequency drifts.

The performance of the frequency stabilizing circuit of Fig. 3 isindicated by the curves of Figs. 8 and 9 in which curve 60 (Fig. 9)indicates the change of oscillator frequency in kilocycles as the platevoltage is varied, where the stabilizing circuit was absent, while curve6| (Fig. 8) shows the change of oscillator frequency in cycles withchanges in plate voltage where the stabilizing circuit was used. Theimprovement in stability obtained was about l50-fold. Plate voltage wastaken as the independent variable because frequency-modulatedoscillators are fundamentally unstable with varying plate voltage. Itwas found that a similar degree of stability was obtained with respectto variations in the setting of tuning condenser over a considerablefrequency range. the frequency of oscillation being determined almostentirely by the tuned circuits 44 and 45.

One disadvantage that the detector plates must be at the same directcurrent potential as the grids'of the control tubes. This necessitatesputting the cathodes of these detector tubes at a negative potentialwith respect to ground suflicient to give the desired detector platevoltage. This disadvantage is obviated in the circuit of Fig. 5 in whichthe triode detectors of Fig. 3 are replaced by diode detectors 62 and63. The circuit of Fig. 5 may be used wherever a diode detector will beof sumcient sensitivity. In this figure the control grid bias issupplied from battery 64 through a choke coil 65. The detected currentsproduce a voltage drop across the respective resistances 66 and 61 whichare applied to the grids of the control tubes for frequencystabilization. These resistances are by-passed by condensers 68 and 69,respectively. By tapping these by-pass condensers across only a part ofthe resistances 66 and 59 it is possible to increase the impedanceacross the tuned circuits and so increase the sharpness of thesecircuits. The invention is not to be construed as limited to theparticular circuits that have been vdismore negative and that a grid oftube 36 more posiof the circuit of Fig. 3 is closed, since these areillustrative, but the scope of the'invention is indicated in the claims.

What is claimed is: 9 1 r 1. A frequency modulation system comprising anoscillator having a parallel resonant circuit for determining thefrequency, a resistance in one branch of said circuit, a grid-controlledtube having a grid circuit coupled to said resistance and a platecircuit effectively across said resonant circuit whereby variations inits plate resistance vary the frequency of oscillations produced, meansto impress-modulating waves upon the grid of said tube, and means tocounteract the tendency of the variations in plate resistance of saidtube to vary the amplitude of the generated oscillations.

2. A frequency modulating system according to claim 1 in which the meansto counteract the .tendency of the variations in plate resistance ofsaid tube to vary the amplitude of the generated oscillations comprisesan impedance in the portion of the plate circuit of said tube that iscommon to its grid circuit.

3. A frequency modulation system comprising an oscillator having aparallel resonant circuit for determining the frequency, connectedbetween the anode and grid, a connection of substantially zero impedanceat the frequency of the generated oscillations from the cathode to anintermediate point of one branch of said parallel resonant circuit, aresistance included in series in said branch between said intermediatepoint and the anode end of said branch, a gridcontrolled discharge tubehaving a grid and having a cathode-anode circuit, means connecting saidgrid to derive voltage variations from across said resistance, meanseil'ectively connecting its cathode-anode impedance across said resonantcircuit whereby variations in its anode-cathode resistance vary thefrequency of oscillations produced, and means to impress modulatingwaves upon the grid circuit of said discharge tube.

4. A frequency modulation system according to claim 3, comprising a loadcircuit coupled to said branch of said resonant circuit between saidintermediate point and the end of the branch to which the oscillatoranode is connected.

5. A frequency modulating system comprising an oscillator having aparallel-resonant frequency-determining circuit effectively connectedbetween the oscillator plate and grid, a low-impedance path for thegenerated oscillations from the cathode to an intermediate point of onebranch of said resonant circuit, a resistance connected to the portionofsaid branch between said intermediate point and the oscillator anode,

a pair of grid-controlled discharge tubes having plate circuits, meanscoupling said plate circuits respectively to portions of said branch onrespectively opposite sides of said intermediate point and inrespectively opposite phase, means connectng the grids of said tubes tosaid resistance an input circuit for modulating waves, means couplingsaid input circuits to said grids in opposite phase, whereby a givenmodulating impulse produces, through the medium of said discharge tubes,frequency varying effects on the oscillation generating circuit that areadditive.

6. In a frequency modulating system, an oscillation generator having aparallel resonant circuit, a resistance connected therein, a pair ofgrid-controlled tubes having plate circuits, means coupling said platecircuits in push-pull relation to one branch of said resonant circuit,means connecting the grids of said tubes in parallel a grid-controlledtube having a cathode and grid and having a plate circuit, meanseffectively coupling said cathode and grid to said resistance, meanseffectively connecting its across said resonant circuit, and means toapply modulating waves to the grid of the latter tube.

8. A modulating system according to claim '7, including a load circuitand means coupling said load circuit to said reactance.

9. In a frequency-modulating system, an 'oscillation generator having afrequency-determining resonant circuit, means to vary the effectivevalue of one of the reactive components of said circuit in accordancewith signal impulses to produce frequency-modulation of the generatedoscillations, means for stabilizing the, mean frequency of theoscillations comprising a pair of tuned detectors tively opposite sidesof said mean frequency, detectors coupled to the resonant circuit andconnected to react on the said means to produce frequency-modulation, insuch direction as to counteract departures from the mean frequency, andmeans preventing said stabilizing means from exerting control of thefrequency except with respect to relatively slow variations in frequencywhereby the stabilizing means does not prevent frequency modulation inaccordance with the signal impulses.

10. In a frequency-modulation system, an oscillation generator having afrequency-determining resonant circuit, a pair of grid-controlled tubeshaving their plate impedances oppositely connected to said circuit,means to impress on their grids voltage waves in approximately phasequadrature to the voltage across the resonant circuit, means to impresssignal waves differentially on said grids, a pair of tuned detectorscoupled to said resonant circuit and having overlapping but displacedresonance characteristics respectively above and below the .meanresonance of the circuit, means to impress the detected currents on saidgrids in a direction to effectively oppose departures from normalfrequency of the generated oscillations, and means preventing control ofthe generated frequency by said detected currents except in response tofrequency variations of less than signal frequency.

11. A system according to claim 10, in which said detectors are simplerectifiers connected across the respective tuned circuits and to therespective grids.

12. A frequency-modulating system comprising an oscillator tube having aresonant circuit one of the reactances of which is included between theplate and cathode of the tube, a resistance connected to said reactanceand included besaid tween the plate and cathode, a pair ofgrid-controlled tubes having their plate impedances oppositely andsymmetrically coupled to the resonant circuit with respect to theoscillator cathode and their grid-cathode circuits connected across saidresistance, a signal input differentially coupled to the grids of saidpair of tubes, whereby the generated oscillations arefrequency-modulated in accordance with signal waves impressed on saidpair of tubes, a pair of tuned plate resistance tuned to frequencies onrespec- 1 means to produce in detectors coupled to said resonant circuitand tuned respectively above and below the mean resonant frequency,means to impress the detected outputs from said detectors on the gridsof said pair of tubes to counteract variations in the frequency of thegenerated oscillations from normal, and filter means in the gridcircuits of said pair of tubes for preventing the counteracting effectsof said detectors except for slower variations inoscillator frequencythan those corresponding to the signal modulation.

13. In a frequency modulating system, an oscillation generator having aresonant frequencydetermining circuit, a source of signal waves, and

said resonant circuit an effective variation in the tuning reactance tovary the frequency of the generated oscillations comprising agrid-controlled space discharge device, means to derive from saidresonant circuit and to impress across two of the electrodes of saiddevice a voltage in quadrature with the generated wave, means to varythe impedance of said device in accordance with the signal waves,thereby producing a tendency toward amplitude modulation of thegenerated oscillations, means to translate the corresponding changes ininternal impedance of said device into changes in the effectivereactance of said resonant circuit and means for counteracting thetendency of the variations in impedance of said device to causeamplitude modulation of the generated oscillations.

14. In a frequency modulating system, an oscillation generator having aresonant frequencydetermining circuit, a source of modulating waves, andmeans to produce in said resonant circuit an effective variation in thetuning reactance to vary the frequency of the generated oscillationscomprising a pair of space discharge tubes having plate circuits, andgrid circuits, means connecting said plate circuits to points ofopposite phase in said resonant circuit, means differentially relatingthe grid circuits of said tubes to said source of modulating waveswhereby their plate impedances are differentially varied by themodulating waves, and means causing the variations in plate impedancesto have cumulative effect on the effective reactance of said reso-' nantcircuit.

15. A wave-length modulation system comprising in combination, means forgenerating wave energy to be modulated comprising an electron dischargedevice including electrodes connected in oscillation generatingcircuits, and means for modulating the length of the wave generated bysaid oscillation generator comprising a reactance tube having an anode,a control electrode, and a cathode, means coupling said anode andcathode with said generating circuits to control the frequency of theoscillations generated, phase displacing means coupling the controlelectrode of said reactance tube to said generating circuits, means forcontrolling the gain of said reactance tube at signal frequency tothereby modulate the length of the generated waves, said anode, controlelectrode and cathode having a common connection to ground, and meanscomprising an impedance included in the connection between said cathodeand ground to oppose modulation of the amplitude of the generatedoscillations in response to the control of the gain of said reactancetube at signal frequency.

16. A wave-length modulation system comprising in combination, anelectron discharge device having electrodes including an anode and acathode coupled in a high frequency alternating current circuit whereinwave energy the wave-length of which is to be modulated is caused toflow, and means for modulating the length of the wave energy flowing insaid circuit comprising a reactance tube having an output electrode, acontrol electrode and a cathode, high frequency coupling means couplingsaid output electrode and cathode of said tube to difierent points onsaid high frequency circuit for impressing between the output electrodeand cathode of said tube high frequency alternating voltage from saidhigh frequency circuit, said tube serving to determine in part thereactance of said high frequency'alternating current circuit and therebycontrol the wave-length of the wave energy flowing in said highfrequency alternating current circuit, a direct current impedance andreactive means for deriving from said high frequency circuit andimpressing 0n the control electrode of said reactance tube a highfrequency voltage which is displaced in phase relative to the highfrequency voltage impressed by said coupling means on the outputelectrode of said tube, and means for controlling the gain of saidreactance tube at signal frequency to thereby modulate the length of thewave energy flowing in said high frequency alternating current circuit.Q

pedance, direct current connection substantially directly connectingtogether said anode and plate, means comprising a direct current elementand a. condenser operating to subject the control electrode of saidreactance tube -to a voltage which is displaced substantially 90 degreeswith respect to the high frequency voltage at a point on said circuithavinginductance and capacity in parallel; means for subjecting thecontrol electrode of said reactance tube to modulatin potentials withrespect to the cathode of said reactance tube to modulate theoscillations in said circuit having inductance and capacity and a loadcircuit coupled to said circuit having inductance and capacity.

OWEN E. DE LANGE.

